When your PC is attacked by 20 rats, it’s a bummer for the DM to make all those attack rolls. The obvious hack is to make one attack roll for the whole group of rats. That gives you pretty spiky results though: the only options are “All the rats hit” or “All the rats miss.”
The 5e DMG has a fix for this: a rule for making monster group (mob) attacks without rolling millions of attack rolls, and without a single all-or-nothing attack roll. You consult a chart which tells you how many attacks hit. For instance, if the monsters hit on a 13, 1/3 of the attackers hit.
I love this idea, and I suggested that it is complete enough to form a whole mass combat system, but after using it in play, I’ve found some problems I’d like to address.
1) There’s no d20 rolls at all. Mob combat is different from any other D&D task resolution.
2) It’s completely smooth. An AC 18 fighter being shot at by 20 hobgoblins is hit by a steady and predictable 5 arrows per round.
3) It requires a chart – not a big one but not one that’s easy to keep in your memory. Like the attack matrix charts in 1e, it’s a page you have to bookmark.
4) Because there are no die rolls, it doesn’t work naturally with advantage/disadvantage and crits are impossible.
Here’s a possible-to-memorize approach, with slightly better math, which allows for misses, variable success, advantage/disadvantage, and crits.
Whenever a group of identical creatures make attack rolls (or any roll really – you could profitably use this for group saving throws too), make a single roll as normal. Divide the creatures into three roughly equal groups. One group rolls this number, one group rolls this with a +5 bonus, and one group rolls this with -5 penalty.
Implications of this system: Advantage/disadvantage doesn’t require any special rules. Just make a single roll with adv/disadv and apply the group modifiers to the result. Auto-miss and crits work as you’d expect too. Because each group uses the same natural die roll, a natural 1 means everyone misses and 20 means everyone crits. That’s fun: the 20 hobgoblins do 40d8+20 (200) damage!
The math: What’s a better model of making 20 attack rolls: this system or the DMG system? Both are pretty good, actually, but mine exactly matches in most situations (whenever you need to roll a 6 to 16 to hit) while the DMG system is better at modeling corner cases (you need to roll a natural 20 to hit or you only miss on a 1). To me that’s not a big deal, because with bounded accuracy, even a bunch of town guards (+3) only need a 16 to hit an adult red dragon (AC 19).
Here’s a chart that compares the average results.
Chance to hit per attack
|d20 roll needed||Rolling all attacks||DMG mob system||blogofholding mob system|
So that’s the system: three groups with +5, +0 and -5 modifiers! Go forth and drown your PCs with armies!